In 1997, the Assembly of the Poor (AOP) successfully organised a 99-day protest, which was widely hailed as ‘a historical moment’ for people's politics in Thailand. However, following the creation of the first Thai Rak Thai government in 2001, the AOP's political role has gradually declined. This thesis aims to investigate the factors behind the AOP's decline between 2001 and 2010. It argues that, because of inherent internal weaknesses and the recent political changes, the development of AOP in the 2000s has increasingly been influenced, if not determined, by external factors. First, the thesis re-examines the movement's internal elements in a more critical view, which evidently contrasts with early writings on the AOP. It argues that some of the AOP's key features, such as its loose structure, are partially to blame for the movement's decline. NGO activists’ roles in the movement are also critically reassessed. More importantly, the thesis also systematically explores the external elements, known as the political opportunity structure (POS), which have increasingly influenced the AOP's development and mobilisation. This approach has been overlooked by most literature written on the movement. The study emphasises two sets of elements of the POS: stable and volatile components. As for stable elements, which are structurally embedded, the list includes the cultural structure, institutional structure, and prevailing strategies. These elements have impeded, not only the AOP, but also other people's movements for decades. On the other hand, the two influential volatile elements for the AOP during this period are the elite divisions and media access. During the Thaksin government, his control over the elites significantly restricted the AOP's campaigns, while the lack of media access had adverse impacts on the movement during this highly polarised period.